"The hair was curiously stirred, as if by breath of hot air; and, though the eyes were wide open, they were perfectly motionless. That, and its livid colour, made it horrible; but its horror seemed to be in spite of the face, and beyond its control, rather than a part of its own expression.
As Scrooge looked fixedly at this phenomenon, it was a knocker again."
What does "phenomenon" mean in this context?